1. Field of the Invention
The present invention relates to films of electrically conductive polymers and the use of such films as electrode material in electrochemical cells, and electrochemical cells containing such films.
2. Discussion of the Background
Over the past few years, the use of electrically conductive polymers as electrode material in primary and secondary cells has become increasingly important. For example, EP-A-36 118 describes electrochemical cells in which one or more electrodes consist of electrically conductive polymers.
The electrically conductive polymers are particularly suitable for use in secondary cells, and it has been possible to show that the charge capacity and the cycle life of some polymers reach values which are of interest in practice (cf. H. Munstedt in H. Kuzmany et al., Electronic Properties of Polymers and Related Compounds, Springer-Verlag, 1985, page 13).
Especially for use in secondary elements, it is necessary for the electrically conductive polymers to be capable of undergoing reversible oxidation. Electrochemical oxidation or reduction is accompanied by the reversible incorporation of counterions into the electrically conductive polymer or removal of these ions from the said polymer. However, all conductive polymers generally have the disadvantage that their conductivity falls with decreasing content of counterions or complexing agents. However, to achieve a high degree of discharge in a secondary cell or in an electrochemical cell, incorporation or removal must be as complete as possible, ideally to the neutral state of the polymer. The very low electrical conductivity of the polymers in the neutral or virtually neutral state necessitates the use of special conductors, as a rule metals or, for example, carbon fibers. This is necessary in order to ensure a conductivity sufficient for the speed of charge transport and hence also to ensure a good power density of the electrochemical cell.
However, there are a number of disadvantages involved in the mounting of special conductors. Many metals exhibit corrosion in the electrolyte solvents used; frequently, the continuous fastening of the conductor to the polymer material of the film over a large area presents difficulties, and, not least, the use of conductors is fairly expensive in terms of construction. The last-mentioned disadvantage is particularly undesirable in large-scale series production.